1. Field of the Invention
The present invention relates to a particle detecting apparatus for a substrate with a film on its surface, and more particularly a particle detecting apparatus which can distinguish and detect a foreign substance on or in the film.
2. Description of the Related Art
With high integration of a semiconductor device, formation of a fine wiring pattern and fine gate pattern is requested. When there is a foreign substance on the pattern, a short circuit is sometimes formed due to the foreign substance. Also, the flatness of an interlayer insulating film is sometimes degraded by the foreign substance so that an upper wiring pattern is broken. With the high integration of the semiconductor device, even if the foreign substance has a small diameter, the foreign substance influences severely on the break of the wiring pattern.
In order to prevent such an influence, a particle detecting method is requested which can detect a particle with a smaller size. The particle with the smaller size could be detected by use of a laser beam with a shorter wavelength. For example, a particle with the size of 0.1 .mu.M can be detected by an Ar laser beam. However, when there is a film of a material such as SiO.sub.2 and SiN formed on a Si substrate, a scattered light beam by the foreign substance is hidden in a scattered light beam by the film, because the laser beam is reflected, absorbed or refracted. As a result, the scattered light beam by the film functions as noise so that it is difficult to detect the foreign substance in a high precision.
It is supposed that the maximum voltage corresponding to a quantity of scattered light beam by the foreign substance is represented by V1 and the maximum voltage of the noise is represented by V2. In this case, the foreign substance cannot be distinguished from the noise, if V1/V2 is not larger than 1 at least. This value is desirably equal to or more than 3. Therefore, the laser wavelength should be changed in accordance with the film thickness and the material of the film. However, it is impossible to cope with problems such as different light absorption and reflectivity depending on the film thickness and the film material only by changing the laser wavelength.
When the wavelength is made short, the light beam quantity transmitting a silicon film formed on a wafer surface sometimes decreases remarkably, so that the existence of a foreign substance particle in the silicon film cannot be detected. A laser unit with at least 2 wavelengths must be used to detect the existence of the foreign substance particle with a small size from the scattered light beam which is influenced by 2 variables of the film thickness and the particle diameter.
A method of detecting a particle by use of such a laser unit with 2 wavelengths is described in Japanese Laid Open Patent Application (JP-A-Heisei 6-148085). In this method, the degradation of the detection precision of a foreign substance particle in the wafer surface due to the interference depending upon the change of the film thickness can be prevented. That is, the method can obtain information relating to the change of the film thickness, but can not obtain information in the direction of the depth in the film. The information which is more important than the film thickness change is existence or non-existence of a foreign substance particle in the film, e.g., a hole in the film. Because the intensity of a light beam scattered by the hole is local but large, it is a detection object which is more important than the film thickness change. When the light beam is scattered by the hole, the film thickness information is not substantively gotten. A method of detecting the existence or non-existence of a foreign substance particle in the substrate surface can be surely is demanded even if the film thickness information is not obtained.